Thank you for the numerous responses! Except the solution to change 
"localhost" to "127.0.0.1" in the whole script, I tried everything you 
proposed. It still doesn't work. Here a short review:

=== Are you trying to connect to the MySQL socket outside of the httpd 
chroot?
===> after having run "apachectl start", I tried the same process using 
"httpd -u". But nothing changed.


=== mysql -h localhost -u root -p
Works perfect. "mysql -h localhost -u joomla -p" works also.

=== Have a look in /var/www/logs/
===>in the errorlog of the folder is no entry. access_log shows up:
172.16.172.130 -- [09/Mar/2010:09:47:26 -0700] "POST" 
/user01/installation/index.php HTTP/1.1" 200 4270

=== At the very least you'll also need the php5-mysql-5.2.6.tgz package 
installed as well.
===> php5-mysql and php5-mysqli packets are installed both

=== At the very least you'll also need the php5-mysql-5.2.6.tgz package 
installed as well.
==> That's the output of the mysql part in the phpinfo();:

mysql

MySQL Support    enabled

active persistent links        0
active links            0
client api version        5.0.51a
mysql_module_type        external
mysql_socket            /var/run/mysql/mysql.sock
mysql_include            -I/usr/local/include/mysql
mysql_libs            -L/usr/local/include/mysql


directive        local value        master value

mysql.allow_persistent    On            On
mysql.connect_timeout    60            60
mysql.default_host    no value        no value
mysql.default_password    no value        no value
mysql.default_port    no value        no value
mysql.default_socket    no value        no value
mysql.default_user    no value        no value
mysql.max_links        Unlimited        Unlimited
mysql.max_persistent    Unlimited        Unlimited
mysql.trace_mode    Off            Off



Thank you!

Jan